Conventionally, various types of condenser microphones (or capacitor microphones) have been developed and produced by way of manufacturing processes of semiconductor devices. A typical example of a condenser microphone includes a diaphragm, which vibrates due to sound waves, and a plate, which is positioned opposite to the diaphragm via a dielectric such as air. An electrostatic capacitance (or capacity) formed by the diaphragm and the plate varies due to vibration of the diaphragm. The condenser microphone converts variations of electrostatic capacitance into electric signals.
Japanese Patent Application Publication No. 2001-518246 teaches a miniature silicon condenser microphone in which a diaphragm is fixed in a cantilever manner. This condenser microphone has a structure in which tensile stress, which occurs during the formation of the diaphragm, does not remain; hence, it is advantageous in that the amplitude of the diaphragm due to sound pressure applied thereto can be increased, whereas the strength of the diaphragm is decreased, so that a stopper is necessary to prevent an excessive amplitude in the diaphragm. This makes the manufacturing process complicated.
Japanese Unexamined Patent Application Publication No. 2002-95093 teaches a condenser microphone in which the peripheral portion of a diaphragm is entirely fixed in position, wherein the overall area of the diaphragm having conductivity is positioned opposite to a plate having conductivity. The peripheral portion of the diaphragm causes very small amplitude so that substantially no variation occurs in capacity, wherein the ratio of the capacity of the peripheral portion of the diaphragm becomes large within the total capacity of the condenser microphone. This degrades the sensitivity of the condenser microphone. Due to the fixing of the peripheral portion of the diaphragm, tensile stress, which remains during the formation of the diaphragm, is not released so that the amplitude of the diaphragm due to sound pressure applied thereto is reduced.
Japanese Unexamined Patent Application Publication No. 2001-231099 teaches an example of an electroacoustic transducer, i.e., a condenser microphone in which four corners of a diaphragm are fixed in position, wherein the diaphragm having conductivity is positioned opposite to a plate having conductivity. The peripheral portion of the diaphragm causes very small amplitude so that substantially no variation in capacity occurs, wherein the ratio of the capacity of the peripheral portion of the diaphragm becomes large within the total capacity of the condenser microphone. This degrades the sensitivity of the condenser microphone. Fixing the four corners of the diaphragm reduces the amplitude of the diaphragm due to sound pressure applied thereto.
Specifically, the aforementioned publication teaches a condenser microphone having a bent portion between the center portion and peripheral portion of a diaphragm. Herein, residual stress applied to the diaphragm is released by way of the deformation of the bent portion so that the amplitude of the diaphragm increases so as to increase the sensitivity. The bent portion is formed using a single thin film, which is generally deposited on the surface of a step portion formed in a sacrifice layer; hence, the thickness thereof becomes small as it approaches the bottom of the step portion because the step portion is difficult to be deposited and is thus reduced in density. That is, the strength of the diaphragm decreases as the height difference of the step portion increases. In other words, it is very difficult to increase the sensitivity by increasing the height difference of the bent portion while maintaining the satisfactory strength of the diaphragm.